"There is not much curry."
no reason, for that english sentence I think is perfectly fine to say「カレーが多くないです」However, japanese people do tend to use 多い or 少ない more than their negative counterparts.
And also for english in particular, if you say "there is not much" is like saying there is very little in quantity, while saying "there is not a lot" is saying that there is some quantity but not enough to be a lot. So that might be the reason for the duolingo distinction.
Because the sentence is really saying something closer to "the curry is few."
The differences between です and あります/います can be confusing, but it helps to consider the intent of the sentence: are you trying to express the way something is (です), or that it exists (あります)?
In this case, we're trying to express the way it is (namely, that it is "few" in amount), hence the use of です.
(Note that this sentence isn't a good example to see these differences in action, because the rules in English for using "not much / a little / few" are wonky compared to Japanese. If we use an adjective that translates better, like 青い (あおい, blue), it makes more sense. Consider:
傘は青いです (かさはあおいです) = The umbrella is blue.
青い傘があります (あおいかさがあります) = There is a blue umbrella.
And it makes a bit more sense.)
少し is an adverb and 少ない is an I-adjective.
日本語を少しだけ話せる ( I only speak a little bit Japanese )
少し食べた ( I ate a little )
友達が少ない ( I don't have a lot of friends )
時間が少なくなった ( There's isn't much time left )